1. Field of the Invention
The present invention relates to an electrostatic image developing toner for use in developing an electrostatic latent image formed by an electrophotographic method or an electrostatic recording method, with a developer; a production process thereof; and a production process of a resin particle liquid dispersion used as a raw material of the toner.
2. Description of the Related Art
At present, a method of visualizing image information through an electrostatic image by an electrophotographic process is being utilized in various fields. In the electrophotographic process, an electrostatic image is formed on a photoreceptor through electrostatic charging and exposure steps, and the electrostatic latent image is developed with a developer containing a toner and then visualized through transfer and fixing steps. The developer used here includes a two-component developer comprising a toner and a carrier, and a one-component developer using a magnetic toner or a non-magnetic toner solely. The toner is generally produced by a kneading and pulverizing production process where a thermoplastic resin is melt-kneaded with a pigment, an electrostatic charge controlling agent and a releasing agent such as wax and after cooling, the kneaded material is finely pulverized and then classified. In such a toner, an inorganic or organic particle is sometimes added to the toner particle surface, if desired, so as to improve flowability or cleaning property.
In recent years, a duplicator, a printer and a complex machine thereof with a facsimile, each employing a color electrophotographic process, are greatly spread. In the case of realizing appropriate gloss in the reproduction of a color image or transparency for obtaining an excellent OHP image, it is generally difficult to use a releasing agent such as wax. Accordingly, a large amount of an oil is applied to a fixing roll so as to assist separation but this causes tacky touch of a duplicated image including an OHP image, makes it difficult to write on the image with a pen or often gives feeling of heterogeneous gloss. In the case of an ordinary black-and-white copy, it is more difficult to use a wax generally employed, such as polyethylene, polypropylene and paraffin, because the OHP transparency is impaired.
Even if, for example, transparency is sacrificed, the wax can be hardly prevented from being exposed to the surface in the conventional production process for toner by a kneading and pulverizing method. As a result, when the toner is used as a developer, there arises a problem such as considerable deterioration in flowability or filming on the developing machine or photoreceptor.
As an ultimate method for overcoming these problems, a production process by a polymerization method is proposed, where an oil phase comprising monomers, which work out to the raw material of a resin, and a colorant is dispersed in an aqueous phase and then directly polymerized to form a toner, thereby enclosing the wax inside the toner and preventing the wax from being exposed to the surface.
Other than this, as a technique of intentionally controlling the shape and surface structure of the toner, a process of producing a toner by an emulsion polymerization and aggregation method is proposed in JP-A-63-282752 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-6-250439. This is a production process where a resin particle liquid dispersion is produced generally by emulsion polymerization or the like, a colorant liquid dispersion is separately produced by dispersing a colorant in a solvent, these liquid dispersions are mixed to form an aggregate having a diameter corresponding to the particle diameter of a toner, and the aggregate particles are fused and coalesced under heating to form a toner.
In such a production process, not only internal inclusion of wax is realized but also reduction in the toner diameter is facilitated and reproduction of a clear image with high resolution is enabled.
In the above-described production process, in order to provide a high-quality image and stably maintain the performance of the toner under various mechanical stresses, it is very important to select the pigment and releasing agent, optimize the amounts thereof, prevent the releasing agent from being exposed to the surface, optimize the resin properties to improve the gloss and releasability without a fixing oil, and suppress the hot offset.
On the other hand, a technique enabling fixing at a lower temperature is demanded to reduce the consumed energy amount and in recent years, it is demanded to stop energizing the fixing machine except for operation so as to attain thorough energy saving. Therefore, the temperature of the fixing machine must be instantaneously elevated to the working temperature upon energization. For this purpose, the heat capacity of the fixing machine is preferably made as small as possible but if the case is so, the fluctuation width of the temperature of the fixing machine tends to be larger than ever. That is, the overshoot of the temperature after start of energization is increased, and the temperature drop due to passing of paper is also increased. Furthermore, when paper in a width smaller than the width of the fixing machine is continuously passed, the temperature difference between the paper passing part and the paper non-passing part becomes large. Particularly, in the case where the fixing machine is used in a high-speed duplicator or printer, such a phenomenon is more liable to occur because the capacity of the power source tends to run short. Therefore, an electrophotographic toner capable of being fixed at a low temperature and broadened in the so-called fixing latitude, that is, free from generation of offset until a high temperature region, is strongly demanded.
As for the technique of decreasing the fixing temperature of the toner, a method where a polycondensation-type crystalline resin showing a sharp melting behavior with respect to the temperature is used as the binder resin constituting the toner is known but in many cases, the crystalline resin cannot be generally used because this resin is difficult to pulverize by a melt-kneading pulverization method.
Also, for the polymerization of a polycondensation-type resin, the reaction must be performed for a long time of 10 hours or more at a high temperature exceeding 200° C. under highly reduced pressure while stirring by a large force, and a large amount of energy is consumed. Therefore, a huge equipment investment is often required for obtaining durability of the reaction equipment.
In the case of producing a toner by an emulsion polymerization and aggregation method as described above, the polycondensation-type crystalline resin polymerized may be emulsified in an aqueous medium to form a latex, aggregated in this state with a pigment, a wax and the like, and then fused and coalesced.
However, the emulsification of the polycondensed resin requires an extremely inefficient and highly energy-consuming step, for example, a step of emulsifying the resin under high shearing at a high temperature exceeding 150° C. or a step of dissolving the resin in a solvent to attain a low viscosity, dispersing the solution in an aqueous medium and then removing the solvent.
Also, the emulsification in an aqueous medium can hardly evade a problem such as hydrolysis, and the design of materials inevitably encounters generation of uncertain factors.
These problems are prominent in a crystalline resin but not limited to a crystalline resin and the same also occurs in the case of a non-crystalline resin.
For example, JP-A-2002-351140 proposes a method for producing a toner for electrostatic image development, wherein a toner raw material containing at least a polyester resin is heated and melted to produce a melt of the toner raw material, the melt is emulsified in an aqueous medium to form resin particles, and the resin particles are aggregated and further coalesced to produce an aggregate of the resin particles.
This method uses a process where using a conventional polycondensation catalyst such as tetrabutyl titanate and using monomers, for example, trimellitic anhydride (TMA) as the polyvalent carboxylic acid, terephthalic acid (TPA) and isophthalic acid (IPA) as the divalent carboxylic acid, polyoxypropylene(2,4)-2,2-bis(4-hydroxyphenyl)propane (BPA-PO) and polyoxyethylene(2,4)-2,2-bis(4-hydroxyphenyl)propane (BPA-EO) as the aromatic diol, and ethylene glycol (EG) as the aliphatic diol, a reaction is performed at 220° C. for 15 hours in a nitrogen stream under atmospheric pressure, the pressure is gradually decreased, a reaction is performed at 10 mmHg to produce a polyester having a weight average molecular weight of about 5,000 to 90,000, the polyester is melt-kneaded with a colorant, a wax and the like, the melt-kneaded product MB 1 is heated to 190° C. and charged into CAVITRON CD1010 (manufactured by Eurotec, Ltd.), 0.5 wt % of dilute ammonia water is added, MB1 is fed to CAVITRON at a rate of 1 L/min under heating at 160° C. by a heat exchanger, and the liquid dispersion slurry obtained after dispersion is cooled to 60° C. and taken out.
For forming a toner, this liquid dispersion is further subjected to aggregation, coalescence, washing and drying. However, such a process apparently requires huge energy at the production and emulsification of the resin and is considered to be unusable in practice.
Furthermore, the emulsification dispersion under such a high energy condition readily incurs decomposition of the resin and causes a problem such as occurrence of uneven distribution of the composition or difficulty in realizing a uniform particle size distribution of resin particles in the liquid dispersion. The toner using such a material readily brings about a problem in the stability of image quality at continuous printing as well as the initial image quality.